Hybrid Energy Storage Systems (HESS) have been developed and implemented as a means to capture, store and redistribute electrical energy for mobile applications during operation. These systems are normally packaged entirely onboard the vehicle and consist of three major subsystems; energy storage, power electronics and system controls. This diffusers from battery electric systems that require a fixed charging station and can only receive and store a charge of energy when not in operation. Historically a HESS used batteries, commonly Nickel Metal Hydride, but in recent years newer battery technologies such as lithium-ion and other alternatives such as Ultra-Capacitors have also been considered.
The primary challenges for successful implementation of a HESS is to develop a reliable, cost effective, package within a given space claim with sufficient storage capacity, charge discharge rate and intuitive user interface to capture, store and redistribute electrical energy. The electrical energy can be provided to the HESS from any regenerative and non regenerative, on board and external power sources. Current use of HESS is primarily in small to medium size on road vehicle applications such as passenger cars and transit busses where the total energy capture, storage and redistribution of electrical energy is relatively small. For example, a city bus weighs approximately 15 tons and will typically be stopping from a speed of 35 mph or less with approximately 0.5 kW-hrs of kinetic energy that could be captured. Implementation of HESS for typical on road applications that provide drive to the wheels through direct mechanical connection requires significant modification to incorporate as there are no electric motor/generators inherent to the drivetrain.
By comparison, a diesel electric locomotive architecture provides drive to the wheels via electric motors powered by an onboard generator driven by a conventional internal combustion engine. This locomotive architecture allows the existing electric “traction” motors connected to the wheels to be back driven as generators to produce electricity while creating resistive braking energy, known as “regenerative” or “dynamic” braking. Current diesel electric locomotives are able to use dynamic braking to slow the train, but the electrical energy produced is dissipated as heat rather than captured and reused. A typical passenger train with 6 bi-level coach cars will weigh 715 tons and typically stop from a speed of 79 mph. Calculations indicate this train will have 112 kW-hrs of kinetic energy available for capture by regenerative braking. This is over 200 times the kinetic energy of the previously described hybrid transit bus. 200 copies of the existing HESS modules for transit buses would not package well on existing rail equipment and would also not stand up well to the more constant steel wheel on steel rail vibration over the 20 year locomotive service life. This much larger amount of energy to store and discharge in short bursts creates difficult packaging and cooling issues to overcome in order to implement a HESS with sufficient storage capacity and service life for passenger rail applications.
Certain rail applications have unique characteristics that provide excellent opportunity to utilize the benefits of a very large HESS. For rail applications the HESS shall be referred to as a Locomotive Energy Storage System (LESS). The primary application for a LESS is for commuter service due to the frequent start and stop nature, high speed and mass of commuter passenger trains. Switcher locomotives are another application where hybridization would be beneficial due to the frequent start and stopping action of the locomotive as it moves small strings of cars back and forth to build up or take apart longer trains.
In the rail application, large enough HESS will require more space than is available on current locomotives. What is desired is a novel packaging of energy storage, power electronics and a control system interface that decreases the per kW-hr cost while adding system redundancy and reliability. This system should be a direct addition to and backwards compatible with, as much as possible, existing train sets and locomotive control systems. It should also be able to incorporate all types of applicable energy storage technologies, ultra capacitor cells, battery cells, etc., in a modular system with adequate provisions to insure safe, reliable and serviceable operation.
Relevant standards include APTA RP-E-014-99 Recommended Practice for Diesel Electric Passenger Locomotive Blended Brake Control, APTA RP-E-016-99 Recommended Practice for 480 VAC Head End Power System, and APTA RP-E-017-99 Recommended Practice for 27-Point Control and Communication Trainlines for Locomotives and Locomotive-Hauled Equipment (by The American Public Transportation Association, 1666 K Street, N. W. Washington, DC, 20006, USA), each of which is incorporated in its entirety as a reference.